A standard form of fine-grained material treatment is disclosed in EP-B-2 054.
Because of the high gas speeds used in order to increase efficiency, the material to be calcined passes through the calcination zone very quickly. Consequently there is little time available for the transfer of heat energy from the fuel to the material to be calcined. If fuels which ignite badly or are slow to burn out are used in the calcination zone, the fuel cannot be fully burnt out in the calcination zone and consequently its heat energy cannot be completely transferred to the material. This results in incomplete burn-out and insufficient deacidification of the material.
The prior art also includes a method (DE-B-22 47 172) in which an adjustable proportion of the material discharged from the lowest cyclone of the preheater is delivered to an additional combustion chamber and is heated there using fuel and exhaust air from the cooler before it then returns to the lowest cyclone of the preheater. Thus with this method an adjustable proportion of the material is deacidified in the additional combustion chamber before it reaches the rotary kiln. However, with this known method it is not reliably ensured that after preheating all particles of the material undergo heating and deacidification in the additional combustion chamber before they reach the rotary kiln for final burning.
A method is also known from DE-C-1 303 507 in which a proportion of the material discharged from the lowest stage of the cyclone preheater is introduced into the exhaust gas pipe leading to this lowest stage and led back to the lowest cyclone stage. Thus with this method a proportion of the preheated material carries out a circuit in the lowest cyclone stage. A calcination zone for heating and deacidification of the preheated material (before it enters the rotary kiln) is not provided in this known method.
The object of the invention is to provide a method and apparatus which function in such a way that even with fuel which ignites badly or burns out with difficulty, and particularly with inferior fuel, an improved burn-out in the calcination zone is achieved as well as a higher degree of deacidification of the material to be calcined.